The invention relates to a display tube for displaying color pictures comprising a display window, a cone and a neck, a display screen, provided on the inside of the display window, comprising a large number of regions luminescing in three different colors, means provided in the neck for generating three electron beams, the beam axes of which are situated substantially in one plane, a color selection electrode positioned before the display screen which assigns each electron beam to luminescent regions of one color, the generating means being provided, on the display screen side, with correction elements placed near each beam, wherein the display tube is meant to cooperate with a system of deflection coils around the transition neck-cone of the display tube to generate a first and a second deflection field, which deflection fields are orthogonal, and the distribution of the first deflection field, on the display screen side, is strongly pin-cushion-shaped and the distribution of the second deflection field, on the display screen side, is strongly barrel-shaped.
Such a display tube is known from the article "Large screen colour television with intrinsically 110.degree. deflection", Electronic Application Bulletin, 33; 2, pp. 75-89. As a result of the strong pin-cushion-shaped and barrel-shaped deflection fields, each electron beam, after deflection, is flattened so that an elliptical target is formed on the display screen. This is termed deflection defocusing (defocusing of the electron beams as a result of the deflection). In order to obtain a good dynamic convergence, which means that the frames of the three electron beams coincide on the whole display screen, it is stated in this publication that it is possible, in principle, to provide the end of the electron gun with correction elements for the local deformation of the deflection field so as to remove the coma error in the dynamic convergence. However, this method does not provide a solution to reduce the effects of the deflection defocusing.
German Patent Application No. 2545718 laid open to public inspection which corresponds with U.S. Pat. No. 4,057,747, also discloses a color display tube in which such correction elements are used. In one of the embodiments described, each electron beam is passed between two strips of a material having a large magnetic permeability. These strips are provided at the end of the electron gun, above and below each electron beam and symmetrically with respect to the plane through the beam axes. In the operating display tube, said strips are situated in the magnetic deflection or convergence fields and deform the field locally so that it becomes barrel-shaped. As a result of this, according to this Patent, the coma error in the dynamic convergence and the deflection defocusing upon deflecting in the horizontal direction are reduced. No solution is given for the deflection defocusing upon deflecting in the vertical direction and the extra deflection defocusing on the picture diagonals. This defocusing is very annoying.